The present invention relates generally to weatherable thermoplastic blends. more specifically, the present invention relates to a novel thermoplastic blend of polycarbonate, polymethylmethacrylate and AES resins with improved environmental stress crack resistance.
Aromatic polycarbonate polymers are well known commercially available materials having a variety of applications in the plastics art. Generally speaking, aromatic polycarbonate resins offer a high resistance to attack by mineral acids, have high tensile strength and high impact strength except in thick sections, good thermal resistance and a dimensional stability far surpassing that of most other thermoplastic materials.
In certain applications the use of aromatic polycarbonates is, however, severely limited due to their relatively poor environmental stress crack resistance to organic solvents such as, for example, gasoline, acetone, heptane, and carbon tetrachloride. Contact with such solvents may occur, for example, when polycarbonates are used in automobiles or when solvents are used to clean or degrease stressed parts made from such resins. The most significant effect of this poor solvent resistance is a loss in vital impact strength and also an increase in brittle type failure of parts which have been exposed to these organic solvents.
Attempts to develop tough, weatherable thermoplastics have resulted in blends of polycarbonate and acrylonitrile-butadiene-styrene (ABS)-type resins, see U.S. Pat. No. 3,130,177; blends of polycarbonate, ABS and multiphase composite acrylic interpolymers, see, U.S. Pat. No. 4,390,657; and blends of polycarbonate, acrylate-styrene-acrylonitrile (ASA) and polymethylmethacrylate (PMMA) resins, see U.S. Pat. No. 4,579,909. However, these blends are not totally satisfactory in general because of either a lack of weatherability or poor resistance to solvents. Polycarbonate-ABS blends, in particular, typically exhibit phase separation (i.e. delamination) in molded parts which results in poor directionality bias in physical properties. In addition, these blends are problematic when used in automotive body parts and fittings because of their poor environmental stress crack resistance to organic solvents such as gasoline.
Thus, it would be advantageous to provide a thermoplastic blend which is resistant to wear and weather, with satisfactory lamination characteristics and physical properties. It would further be advantageous if the thermoplastic blend exhibited good environmental stress crack resistance toward organic solvents such as gasoline.